The subject matter of the present invention relates generally to security systems having a large number of detectors for detecting different conditions such as smoke, fire or intrusion of unauthorized persons. They may include switch-type detectors for sensing window breakage, door opening, window opening, floor mat sensors, sound detectors and radiant energy type detectors, including those using infrared, ultrasonics, microwaves or visible light. In a typical complex security installation a building will be provided with twenty such detectors per floor. In order to reduce system cost and circuit complexity, a plurality of sensor loops are employed, each loop including a plurality of detectors and associated identification means for identifying which detector in the loop has been actuated by producing analog detection signals of different amplitudes for each detector.
In U.S. Pat. No. 4,310,835 of Sefton issued Jan. 12, 1982, a security system is disclosed having sensor loops which each include a plurality of detectors connected in series. The detectors in the sensor loop are each connected in parallel with resistors of equal value which indicate the actuation of a detector by a change in the resistance of the sensor loop but do not identify the actuated detector. Thus, the outputs of the sensor loops are connected to a central control circuit containing a monostable circuit formed by a pair of NOR gates which is triggered by the detector signal to detect changes in resistance of the sensor loop due to the actuation of one of the detector switches. However, since the resistances in parallel with the detector switches are of the same resistance value, there is no way of identifying which detector is actuated so that security personnel cannot know whether to respond immediately or how to respond without viewing the detector. For example, if one of the detectors in the loop was a fire detector while another detector was an open door switch, there would be no way of knowing whether there was a fire or the door had been inadvertently left open.
The security system of the present invention overcomes this problem by providing an identification means for the detectors in the sensor loop including a separate identification means for each of the detectors to identify the actuated detector by changes in the amplitude of the analog detector signal produced by such loop. This type of analog signal identification means is much simplier and less expensive than that used by prior systems employing digital coded identification signals which have previously been employed to identify different detectors by sending such digital coded identification signals over the same line as the detector signal, but at a different time.
It has been suggested previously in U.S. Pat. No. 4,118,700 of Lenihan issued Oct. 3, 1978 to provide a security system including a single sensor loop having a plurality of detectors connected in series, such detectors being connected in parallel with resistors of different value in order to identify the detectors actuated by changes in the amplitude of the analog detection signal similar to the present invention. However, the analog signal output of the sensor loop is connected to one input of five voltage comparators, each having their other inputs connected to different D.C. reference voltages. The comparators are switching circuits having their outputs connected through NAND gates to provide an analog to digital converter having a digital detection signal output. This analog to digital converter having a single analog input and a plurality of comparators, is much more complicated and has many more circuit components than the security system of the present invention. The security system of the present innvention is of reduced complexity and greater flexibility due to employing an analog to digital con- verter and multiplexer circuit having a plurality of inputs which are connected to a plurality of different sensor loops. The multiplexer selectively switches between such inputs and applies them to the same comparator at different times to convert the analog detector signal of the selected loop to a digital detector output signal.
Also, the outputs of the analog to digital converter in the present security system are connected to a digital data processing circuit, such as a microprocessor computer, which processes the digital detector signals to determine which actuated detector in the loop generated such signals and actuates an appropriate alarm such as a bell or light indicator. The digital data processor controls the operation of the multiplexer and the analog to digital converter. One suitable analog to digital converter and multiplexer circuit has sixteen inputs which may be connected to sixteen sensor loops, each loop having four or more detecors so that the security system is capable of sensing sixty-four or more detectors. None of the prior security systems use such a multiplexer and analog to digital converter controlled by a digital data processor or microprocessor computer.
Further, the security system of the present invention differs from such prior systems by employing sensor loops having detectors connected in parallel sensor circuits. As a result, such detectors may be more easily connected to the loop by "insulation displacement" connections which penetrate through the insulating coating on the loop wire to form an electrical connection mechanically without stripping the insulation from such wire. Thus, the parallel connection of the detectors enables such detectors to be more easily and quickly installed. Also, such connections are more reliable and do not break as easily as those formed by twisted wires or soldering.
Additional flexibility results in the present security system by using a microprocessor computer to control the analog to digital converter because of the ability to change the computer program of the microprocessor by means of a keyboard directly connected thereto or by a central station having a remote keyboard and display connected by phone lines thereto. As a result, faulty detectors can be ignored or entire sensor loops deactivated without rewiring by changing the computer program, or the program may be modified to accommodate additional sensor loops or additional detectors. No such flexibility is possible in the previous security systems because none of them employ a microprocessor computer or other similar digital data processing circuitry.